HRSG User's Group: Improving steam-plant reliability, durability and profitability    

Dye-Penetrant Testing

Joe Frey, Joe W. Frey Engineering Associates, LLC

Dye-penetrant testing is one of the most popular non-destructive evaluation (NDE) techniques in power plants, because it can identify cracks and other defects in the smooth surfaces of all metals—including both magnetic steel and non-magnetic steel—as well as of most non-metallic materials (Fig 1).

Figure 1: Incipient defects can be identified with dye-penetrant test, long before they develop into a complete fracture.

“But there’s more to it than just spray it, wipe it, and look at it,” Joe Frey explained in this presentation.  Frey—a seasoned engineer in both combined-cycle and coal-fired plants, and a member of numerous code-writing committees—started his presentation with various updates on the latest American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, that HRSG users need to know.

Frey then moved on to the ASME Code’s seven steps for a dye-penetrant test, which are:

    1. Clean the surface.
    2. Apply the penetrant.
    3. Wait for the penetrant to seep into the defects, if there are any. This ‘dwell time’ is prescribed in the code for each of the different types of materials being tested—weldments, castings, forgings, etc.
    4. Wipe off the excess penetrant.
    5. Apply the developer—a different chemical compound that draws the raw penetrant seeped into defects back up to the surface where it will become visible to the inspector’s eye, thus revealing the defects.
    6. Wait for the developer’s prescribed dwell time. 
    7. Interpret the results (look at it).

To make sure these steps are followed properly, Frey said, you need to hire a qualified inspector. The qualification of inspectors also is prescribed in the ASME code, for Level I, Level II, and Level III inspectors, each of whom must complete specific training and practical experience, and pass a three-part exam covering general questions about dye-penetrant testing, as well as specific questions pertaining to different industries—airlines, power plants, etc. To remain qualified, an inspector must perform a dye-penetrant test every five years, and must have an eye test every year. 

The most common mistakes that Frey has seen inspectors make are: 

    1. Shortcutting the dwell times—everybody’s in a hurry these days, right?
    2. Not providing a thorough, written report of the test results. The ASME code is clear about the written report, too, prescribing that the following information must be documented:
        • Which ASME code the inspector followed—its publication date, and name.
        • Type of penetrant—Type I; Fluorescent penetrants, which contain dyes that fluoresce when viewed in a dark room under an ultraviolet light; or Type II, Visible penetrants, which contain a red dye that doesn’t require a dark room and an ultraviolet light to see. Both types are effective, Frey noted, but he prefers Visible penetrants because they’re less vulnerable to contamination from things like cleaning fluid.
        • A map or other indication of the test-piece’s location in the plant.
        • The test-piece’s material and thickness.
        • Lighting equipment.

A thorough, written report is needed, Frey emphasized, not just to keep your administrative records in-shape. It’s also needed in case your dye-penetrant test fails to find an existing defect, and then that test-piece later fractures when it’s re-installed in the HRSG and returned to service. He gave an actual case history where this happened—A test-piece fractured shortly after it had been declared defect-free by a dye-penetrant test. Metallurgists who subsequently investigated that fracture were able to determine from the written report that a through-wall crack had been present at the time of the test, but the NDE inspector didn’t find it because he hadn’t performed his test properly. (Fig. 2)

Figure 2: The dye penetrant (the pink compound) has been applied to this joint, and will seep into the pipe, wherever a microscopic defect exists.

Frey’s concluding advice to HRSG users was “Get to know your NDE inspector, because, for lean-staffed combined-cycle plants, he’s going to be an outsourced contractor. And you’ll want to hire somebody you know and trust.”